This invention relates to a method for making a solid electrolyte capacitor and more particularly to such a method that includes forming a temporary solid mask in the porous capacitor anode.
Solid electrolyte capacitors are made by compressing a valve metal powder, usually tantalum, and sintering to produce a porous pellet. An anode riser wire is attached to the pellet. A dielectric film consisting of the valve metal oxide is formed over the surfaces of the porous pellet, usually by an electrochemical forming step. A solid electrolyte is then produced over the dielectric oxide film; this being accomplished in most instances by holding the pellet by the anode wire, dipping it briefly into a solution of manganous salt and pyrolyzing at a temperature from about 280.degree. to 400.degree. C to convert the salt in situ to manganese dioxide. Finally, a counterelectrode layer is deposited over the semiconductive manganese dioxide to serve as the cathode connecting means for the capacitor.
It is important in the above process to assure that the manganese dioxide solid electrolyte does not contact the extending anode wire which would lead to a shorted capacitor or at best would lead to abnormally high leakage currents.
It is known to apply an insulative coating, e.g. of polytetrafluoroethylene or of an epoxy resin, to the anode wire to prevent contact of the manganous salt to the wire. The coating must be applied either with great care or by masking to prevent the inadvertant introduction of the insulative material into the pores of the pellet which leads to a reduction of the electrical capacity and degradation of the fragile dielectric film in regions of the pellet that are adjacent to the anode wire. The latter often leads to high dissipation factors and increased leakage currents. Known liquid masking materials for this purpose are easily displaced during handling and during application of the insulative coating and consequently do not provide a reliable masking means.
It is therefore an object of this invention to provide a reliable pellet masking procedure in a method for making an electrolytic capacitor.
It is a further object of this invention to provide a relatively non critical low cost method for manufacturing solid electrolyte capacitors with high yields.